What Size Generator Do I Need? A Complete Sizing Guide for Home Backup
- by Joe Weber - updated on 12/22/2025
Key Takeaways
- Know Your Watts: Understanding the difference between starting watts (surge) and running watts (continuous) is the most critical step in sizing a generator.
- Calculate Precision: You don't need a generator that powers everything at once; you need one that handles your total running load plus your single highest starting load.
- Fuel Matters: Dual-fuel generators (gas and propane) offer flexibility during long-term power outages when gasoline may be scarce.
- Silent Options: For smaller needs or sensitive electronics, battery power stations (like Goal Zero) or inverter generators are quieter, cleaner alternatives to traditional gas units.
Power outages are more than just an inconvenience—they can spoil food, cut off heat or AC, and leave your family in the dark. Whether it's a summer storm damaging the power grid or a winter freeze snapping lines, having a reliable home backup power solution is essential for safety and comfort.
But buying a generator isn't "one size fits all." A unit that is too small will trip its breakers and fail to power your essentials, while a unit that is too large burns unnecessary fuel and costs more upfront. This guide will help you answer the critical question: "What size generator do I need?" so you can weather any storm with confidence.
The Basics: Starting Watts vs. Running Watts
To accurately size a generator, you must understand the two types of energy your appliances use.
- Running Watts (Continuous Watts): This is the amount of electricity an appliance needs to keep operating. For example, a refrigerator might need 700 watts to keep the compressor running.
- Starting Watts (Surge Watts): This is the burst of energy required to start a motor-driven appliance. That same refrigerator might need 2,200 watts for a split second to kick the compressor into gear.
Pro Tip: If you ignore starting watts, your generator will likely overload the moment your AC or fridge kicks on, even if the generator has enough "running" power.
Step-by-Step: How to Calculate Your Generator Size
Follow this simple 3-step process to find your "magic number" for generator sizing.
Step 1: Identify Your Essentials
Make a list of the appliances you must have running during a power outage. Common essentials include:
- Refrigerator/Freezer
- Sump Pump
- Furnace Fan (for heat)
- Window AC Unit
- Lights and Device Chargers
Step 2: Determine Wattage
Look for the data tag or label on each appliance to find its voltage and amperage (Watts = Volts x Amps) or look for the stated wattage. Note both the running and starting watts.
Step 3: Do the Math
There are two ways to calculate this.
- The "Total Capability" Method (Safest): Add up the starting watts of every single appliance. This ensures you can turn everything on simultaneously without issue. However, this often results in buying a much larger generator than necessary.
- The "Smart Management" Method (Efficient): Add up all the running watts of your appliances. Then, add only the highest single starting watt figure from your list. This calculation assumes you won't start your AC, sump pump, and fridge at the exact same millisecond.
Example Sizing Chart
Here is a realistic power requirement scenario for a standard home:
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| 12/HP Sump Pump | 1,050 | 2,200 |
| TV / Lights | 500 | 0 |
| Furnace Fan | 800 | 2,350 |
| Central AC (Small) | 2,500 | 4,500 |
| Totals | 5,550 | (Max Surge: 4,500) |
Calculation:
- Total Running Load: 5,550 Watts
- Highest Surge Requirement: 4,500 Watts (The AC unit)
- Recommended Generator Size: You need a generator that provides at least 5,550 running watts. However, to handle the AC starting up while everything else is running, you need enough surge capacity.
Note: Always leave a 10-20% buffer on your generator's capacity to ensure it doesn't run at 100% load, which prolongs its life.
Types of Generators: Which is Right for You?
Once you know your wattage, choose the form factor that fits your lifestyle.
1. Portable Generators (Gas/Propane)
These are the workhorses of emergency power. Brands like Champion offer rugged, high-output generators ideal for running multiple large appliances.
- Pros: High power output (up to 12,000+ watts), cost-effective.
- Cons: Louder, requires regular maintenance.
- Feature to Look For: Dual-Fuel Technology. This allows you to run the generator on propane, which has an indefinite shelf life compared to gasoline.
2. Inverter Generators
Champion inverter generators produce "clean" power that is safe for sensitive electronics like laptops and medical devices. They adjust their engine speed to the load, making them much quieter and fuel-efficient.
- Best For: Camping, tailgating, and powering specific home essentials (fridge + lights).
- Feature to Look For: Parallel Capability. You can link two smaller inverters together to double your power output.
3. Solar Generators (Lithium Power Stations)
For a silent, fume-free solution, look to Solar Generators from premium brands like X2Power and Goal Zero. These units utilize advanced lithium battery technology to store massive amounts of power in a portable frame.
- How They Work: You can charge them via a wall outlet before a storm, or connect them to solar panels to recharge effectively when no outlets are available.
- Best For: Apartments, indoor use (they produce no exhaust), CPAP machines, and keeping phones/laptops charged during power outages.
- Pros: 100% silent, safe for indoor use, push-button start, and renewable charging via the sun.
- Cons: Runtime is limited by battery capacity; ideal for electronics and small appliances rather than whole-home HVAC.
4. Home Standby Generators
For the ultimate peace of mind, a Home Standby Generator is a permanently installed unit that sits outside your house (similar to an AC compressor).
- How They Work: They are connected directly to your home's natural gas or liquid propane supply. When the power goes out, an Automatic Transfer Switch (ATS) detects the outage and turns the generator on automatically within seconds—whether you are home or not.
- Best For: Homeowners who want a "hands-off" solution that powers everything, including central air conditioning and large appliances.
- Pros: Automatic operation, no need to refill gas cans (runs on home fuel supply), typically quieter than portable units.
- Cons: Much higher upfront cost and requires professional installation by an electrician.
Top Features to Consider
When shopping for a generator at retailers like Batteries Plus, look for these convenience features:
- Remote Start: Wirelessly start your generator from inside your warm house with a key fob.
- CO Shield / Sensor: A vital safety feature that shuts the generator off if dangerous levels of carbon monoxide are detected.
- Wheel Kits: Generators are heavy; never underestimate the value of good "never-flat" tires.
Not Sure Where to Start? Visit the Experts in Charge.
Calculating wattage and choosing between inverters, gas beasts, and solar generators can be complicated. Don't guess when it comes to your family's safety.
Visit your nearest Batteries Plus store today. Our in-store experts can help you:
- Double-check your wattage math to ensure you buy the right size.
- Demonstrate the differences between X2Power lithium stations and traditional gas units.
- Recycle your old batteries and bulbs while you shop.
Stop by today and let the Experts in Charge help you weather the storm.
Frequently Asked Questions (FAQs)
Q: Can I run my whole house on a portable generator?
A: It depends on the size of the generator and your home. A large portable generator (10,000+ watts) can power most household essentials, including a central AC and water heater, provided it is connected via a transfer switch. It generally cannot power everything (like electric ranges and dryers) simultaneously.
Q: What happens if I overload my generator?
A: If you exceed the running or starting watts, the generator's circuit breaker will trip to protect the unit, cutting off power. Repeated overloading can damage both the generator and your appliances.
Q: Can I use a solar generator inside?
A: Yes! Unlike gas generators, which produce deadly carbon monoxide, solar generators (like X2Power and Goal Zero) are battery-powered and emit no fumes, making them perfectly safe for use inside your home, apartment, or tent.
Q: How do I connect a generator to my house?
A: You can run extension cords directly from the generator to your appliances (easiest method). For a whole-home solution, a qualified electrician must install a transfer switch. Never plug a generator directly into a wall outlet (backfeeding), as this is illegal in most areas and dangerous to utility workers.
Q: What is a transfer switch?
A: A transfer switch is a safety device installed by an electrician that connects your generator directly to your home's circuit panel. This allows you to power hardwired appliances (like a furnace or well pump) without using extension cords. Most importantly, it prevents dangerous "backfeeding" of electricity into the power lines, keeping utility workers safe while they repair the grid.